Process for manufacturing cold sealed spark plugs

ABSTRACT

Insulators for spark plugs are pressed into an interference fit with steel shells to provide good heat transfer from the insulator tip to the shell. The interference fit is produced by deforming a portion of the shell inward to form an inwardly projecting ridge of the shell metal, and then pressing a portion of the insulator into an interference fit with the ridge. A polymeric sealer is applied to the insulator shoulder adjacent the upper edge of the shell, and the shell edge is rolled over the sealer to extrude a bead of sealer outward between the shell edge and the insulator. In an alternate construction, the insulator is tapered so only its upper portion has an interference fit the with shell.

C Umted States Patent [m 3,594,883

[72] Inventor Stewart V. Bray 1,433,149 10/1922 Premery 313/143 X Allen Park, Mich. 1,753,833 4/1930 Mueller.. 29/51 1 [21] Appl. No. 800,595 2,174,362 9/1939 Doran 29/25.12 X [22] Filed Feb. 19,1969 2,312,103 2/1943 Kushler 29/25.12 [45] Patented July 27, 1971 2,499,823 3/1950 Gogel 29/25.l2 X [73] Assignee Ford Motor Company 3,087,747 4/1963 Novotny. 29/525 Dearborn, Mich. 3,451,110 10/1966 Bray 29/25.12 Contin tion-i rt of 0 583,583? on. T966, :tiif ifii ni No "'"T' f"T' Campbc" 3 "0 Assistant lzxammer Rlchard Bernard Lazarus A1mr -ys --Juhn R. Faulkner um! (ilcnn S. Arcndsen [S4] PROCESS FOR MANUFACTURING COLD SEALED g 'tz s kgg ABSTRACT: insulators for spark plugs are pressed into an interference fit with steel shells to provide good heat transfer [52] U-S-Ci... 29/25.l2, I from the insulator tip to the helL The interference fit i3 313/143 produced by deforming a portion of the shell inward to form [51] lnt.Cl ..F23q 3/70, an inwardly projecting ridge of the shell metal, and then H0it 13/00 pressing a portion of the insulator into an interference fit with [50] Field 0 Search 29/25.i2; thg ridge A olymeric ealer is applied [0 the insulator 29/51 525 shoulder adjacent the upper edge of the shell, and the shell 1 References Cited edge is rolled over the sealer to extrude a bead of sealer outward between the shell edge and the insulator. in an alternate UNITED STATES PATENTS construction, the insulator is tapered so only its upper portion 1,264,870 4/1918 Grainger ,26 i L 36 J6 i 34 30 i i PROCESS FOR MANUFACTURING COLD SEALED SPARK PLUGS This application is a continuation-in-part of US. Pat. application No. 583,580 filed Oct. 3, 1966, now US. Pat. No. 3,'45I,1l0.

BACKGROUND OF THE INVENTION Most commercially available spark plugs are manufactured by a 'hot-presssing operation designed to seal the surfaces between the spark plug shell and the insulator. The hot pressing operation generally is carried out by assembling the insulator into the shell, heating at least a portion of the shell to a temperature of about 1,500 F. and then pressing the shell onto the insulator. As the shell cools, its thermal contraction clamps the pressed area onto the insulator. During spark plug operation at temperatures above about 575 F., thermal expansion relieves the clamping force of the shell on the insulator, however, and the plug begins to leak combustion gases. in addition this hot-pressing process cannot be used for decorative shells such as those plated with chromium, nickel, zinc or cadmium since the high temperatures will destroy the decoration. Some commercial spark plus are made by a cold process in which the insulator is assembled loosely into the shell and powder is pressed into the space between the insulator and shell to effect a seal. Several pressing operations are necessary to achieve the compaction of the powder necessary to produce an effective seal.

SUMMARY OF THE INVENTION This invention provides a process for sealing the surface between the spark plug insulator and shell at room temperature without necessitating multiple compaction steps or handling sealing powders. The process-comprises deforming at least a portion of the steel shell so the outside diameter of at least the corresponding portion of the central insulator is significantly greater than the interior diameter of the deformed portion. A deformation having a substantially flat inner portion with curved ends preferably is used. The insulator then is forced into place within the steel shell by means of an axially exerted force to generate a radial interference fit between the insulator and the deformed portion of the steel shell. A permanent elastic stress is produced in the steel shell regardless of the subsequent temperatures to which the spark plug is exposed and the interference fit assists in transferring heat from the insulator to the shell.

After shell deformation butbefore the insulator has been pressed therein, the exterior service of the shell can be plated or otherwise decorated. The deformation preferably is located in' the portion of the shell adjacent the lower end of the massive portion of the insulator to provide a relatively short path for the heat traveling from the insulator tip. After theinsulator has been pressed into the shell a polymeric sealing material such as a silicone rubber or an epoxy is flowed into the recess 'located between the upper portion of the insulator and the upper part of the shell, the upper part of the shell is rolled over the sealer and the sealer is cured. The shell preferably is rolled over the sealer in a manner extruding a bead of the sealer out between the shell edge and the insulator, which places a high resistance in the corona discharge path from the exterior connection of the plug to the shell. Some of the more elastic sealers can be cured before the edge is rolled over onto the sealer.

In an alternate: process, an interference fit is designed to just below the interference fit. After the insulator has been pressed into the shell, additional sealer is applied above the interference fit and the upper edge of the shell is rolled over the sealer.

2 BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION Referring to FIG. 1, the steel shell 10 of a spark plug has a substantially cylindrical shape and contains a relatively large axial passage I2. Screw threads I4 are formed on. the lower portion of the shell to permit assembling the shell into the engine. The large diameter of passage 12 is reduced by a shoulder 16 into a smaller passage 18 that is surrounded by I threads 14. A relatively wide groove 20 is formed in the exterior surface of shell 10 a short distance above shoulder 16.

After shell 10 has been formed as shown in FIG. I, a compressive force is applied to groove 20 that is sufficient to deform inwardly the metal at groove 20 to achieve the shape shown in FIG. 2. The cross section of the deformed metal preferably has a substantially flat central portion 22 that curves smoothly as at 24 into the undeformed metal of shell 10 on either side. A deformation sufficient to establish a radial interference fit of 0.005 to 0.020 inches with the ceramic insulator intended for use in the shell preferably is used.

A ceramic insulator 26 (FIG. 3) having a tip portion 27, a massive portion 28, and an exterior portion 29 is located in passage 12 with tip portion 27 projecting into passage 18.

Massive portion 28 has two bands 29 and 30 of slightly larger radius on its lower and upper ends respectively and the bands slide easily in passage 12 until the lower band contacts the deformed metal at groove 20. An axial compressive force of about 3 tons is then applied to the insulator in the direction of arrows 31. Under this force, and 29 of the massive portion is pressed intoa radial interference fit with the deformed metal at groove 20. The radial interference fit produces a permanent elastic stress inshell 10 at groove 20. A metal gasket 32 can be positioned on shoulder vl6 before the insulator is inserted if desired; the lower shoulder of the massive portion of the insulator then seats on gasket 32 after the insulator has been pressed into the shell. I

A bead of a flowable-polymeric sealer. 34 is applied to the portion of the insulator above band 30 adjacent the upper edge 36 of shell 10. A typical sealer is a silicone rubber having a high-dielectric strength obtainable from General Electric, Dow-Corning, or Staufier Chemical Co. Upper edge 36 of shell 10 is rolled over the sealer 34 in a manner such that part of the sealer extrudes between edge 36 and exterior portion 29 of the insulator to form an externally exposed bead 38 as shown in FIG. 3. Prior to rolling edge 36 over the sealer, the sealer can be cured partially by a blast of hot air if desired. After rolling, sealer curing can be completed rapidly by induction heating the proper portion of the assembly or by passing. the assembly through an oven. 7

During spark plug operation, heat from tip portion 27 flows readily into band 29 and through the intimate contact of band 29 with the deformed metal at groove 20. Corona discharge from the exterior electrical connection to the plug at the top of exterior portion 29 is reduced considerablv by the high electrical resistance of bead 38 projecting between portion 29 and edge 36.

Turning to the alternate construction of FIG. 4, groove 20 and the deforming step are eliminated in preparing shell 10a The massive portion 28a of insulator 26a is tapered inwardly from exterior portion 29a toward tip portion 270 so the lower part of the massive portion slides easily into shell 10a but the upper part has the interference fit. A radial groove 40 is formed in the upper part of the massive portion just below the area of the interference fit.

A metal gasket 32a is located on shoulder 16a, and groove 40 is filled with a sealer 42. Sealer 42 can be the silicone rubber as described above, A high axial-compressive force then is applied to insulator 26a in the direction of arrows 31 to force the insulator into the shell. The force produces the permanent interference fit between the area of massive portion 28a above groove 42 and the corresponding area of shell a.

Sealer 34a then is applied to the insulator adjacent edge 36a and edge 36a is rolled over sealer 34a in a manner extruding part of the sealer between edge 36 and portion 290 of the insulator as described above. The assembly then can be heated in an oven to cure sealers 34a and 42.

The high interference fit at the upper area of massive portion 28a seals the insulator to the shell and removes heat from the insulator with sufficient efficiency to prevent excessive temperatures at sealer 42 or sealer 34a. Gasket 16 prevents combustion gases from reaching sealer 42 directly.

Thus this invention provides a cold-sealed spark plug that uses a permanent radial interference fit between the ceramic insulator and the steel shell to seal the insulator-shell surfaces and assist in transferring heat from the insulator. Spark plugs produced by the process do not leak at temperatures exceeding 675 F. The permanent interference fit ensures proper sealing and heat transfer at all anticipated operating temperatures throughout the life of the spark plug.

I claim:

1. A process for manufacturing spark plugs comprising; preparing a central insulator and a complementary steel shell for receiving the central insulator, deforming at least a portion of the steel shell so the corresponding portion of the central insulator is significantly greater than the interior diameter of the deformed portion of the steel shell, forcing the central insulator into place within the steel shell by means of an axially exerted force whereby a radial interference fit is established between the central insulator and the deformed portion of the steel shell and permanent elastic stress is induced in the steel shell, said radial interference fit assisting in transferring heat from the insulator to the shell dispensing a flowable sealer onto the upper shoulder of the insulator after said insulator has been pressed into the shell, curing the sealer and rolling the upper edge of the shell over the sealer, and extruding part of the sealer out between the insulator and the upper edge of the shell when rolling the upper edge of the shell to dispose the extruded sealer between the rolled shell edge and the insulator.

2. The process of claim 1 inwhich the deformed portion of the steel shell has a cross section curved at its ends and substantially flat in the center and is located near the lower end of the massive portion of the insulator.

3. A process for manufacturing spark plugs comprising; providing a central ceramic insulator and a complementary steel shell receiving said insulator, said insulator being tapered so its lower portion has a diameter smaller than the interior diameter of the shell and its upper portion has a diameter greater than the interior diameter of the shell, forming a groove in the upper portion of said insulator below the maximum diameter of the upper portion, filling said groove with a sealer, forcing the insulator into place within the shell by exerting an axial force whereby a radial interference fit is established between the upper portion of the insulator and the shell and permanent elastic stress is induced in the shell, said radial interference fit assisting in transferring heat from the insulator to the shell, dispensing a flowable sealer onto the upper shoulder of the insulator after said insulator has been pressed into the shell, rolling the upper edge of the shell over the sealer and extruding part of the sealer out between the insulator and the upper edge of the shell to dispose the extruded sealer between the rolled shelled edge and the insulator, and curing the sealer. 

1. A process for manufacturing spark plugs comprising; preparing a central insulator and a complementary steel shell for receiving the central insulator, deforming at least a portion of the steel shell so the corresponding portion of the central insulator is significantly greater than the interior diameter of the deformed portion of the steel shell, forcing the central insulator into place within the steel shell by means of an axially exerted force whereby a radial interference fit is established between the central insulator and the deformed portion of the steel shell and permanent elastic stress is induced in the steel shell, said radial interference fit assisting in transferring heat from the insulator to the shell dispensing a flowable sealer onto the upper shoulder of the insulator after said insulator has been pressed into the shell, curing the sealer and rolling the upper edge of the shell over the sealer, and extruding part Of the sealer out between the insulator and the upper edge of the shell when rolling the upper edge of the shell to dispose the extruded sealer between the rolled shell edge and the insulator.
 2. The process of claim 1 in which the deformed portion of the steel shell has a cross section curved at its ends and substantially flat in the center and is located near the lower end of the massive portion of the insulator.
 3. A process for manufacturing spark plugs comprising; providing a central ceramic insulator and a complementary steel shell receiving said insulator, said insulator being tapered so its lower portion has a diameter smaller than the interior diameter of the shell and its upper portion has a diameter greater than the interior diameter of the shell, forming a groove in the upper portion of said insulator below the maximum diameter of the upper portion, filling said groove with a sealer, forcing the insulator into place within the shell by exerting an axial force whereby a radial interference fit is established between the upper portion of the insulator and the shell and permanent elastic stress is induced in the shell, said radial interference fit assisting in transferring heat from the insulator to the shell, dispensing a flowable sealer onto the upper shoulder of the insulator after said insulator has been pressed into the shell, rolling the upper edge of the shell over the sealer and extruding part of the sealer out between the insulator and the upper edge of the shell to dispose the extruded sealer between the rolled shelled edge and the insulator, and curing the sealer. 